Portable devices such as mobile telephones, laptop computers, and the like often utilize rechargeable energy storage devices such as rechargeable batteries as a source of power. From time to time, the battery of a portable device will run low and a user will not have access to a charger to recharge the battery. This may be the case, for example, if a user is commuting on a transit vehicle. However, as such portable devices are becoming more and more ubiquitous, the user may have access to another portable device with ample energy.